Freestanding reactive multilayer foils

ABSTRACT

Reactive foils and their uses are provided as localized heat sources useful, for example, in ignition, joining and propulsion. An improved reactive foil is preferably a freestanding multilayered foil structure made up of alternating layers selected from materials that will react with one another in an exothermic and self-propagating reaction. Upon reacting, this foil supplies highly localized heat energy that may be applied, for example, to joining layers, or directly to bulk materials that are to be joined. This foil heat-source allows rapid bonding to occur at room temperature in virtually any environment (e.g., air, vacuum, water, etc.). If a joining material is used, the foil reaction will supply enough heat to melt the joining materials, which upon cooling will form a strong bond, joining two or more bulk materials. If no joining material is used, the foil reaction supplies heat directly to at least two bulk materials, melting a portion of each bulk, which upon cooling, form a strong bond. Additionally, the foil may be designed with openings that allow extrusion of the joining (or bulk) material through the foil to enhance bonding.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of United States ProvisionalApplication Serial No. 60/201,292 filed by the present applicants on May2, 2000 and entitled “Reactive Multilayer Foils”. It is related to U.S.Application Ser. No. ______ filed by M. E. Reiss et al. concurrentlyherewith and entitled “Method of Making Reactive Multilayer Foil andResulting Product” and U.S. Application Ser. No. filed by ______ T. P.Weihs et al. concurrently herewith and entitled “Reactive MultilayerStructures For Ease of Processing and Enhanced Ductility”. These threerelated applications are incorporated herein by reference.

GOVERNMENT INTEREST

[0002] This invention was made with government support under NSF GrantNos. DMR-9702546 and DMR-9632526 and The Army Research Lab/AdvancedMaterials Characterization Program through Award No. 019620047. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention relates to reactive multilayer foils, especiallyfreestanding multilayer foils, useful as local heat sources.

BACKGROUND OF THE INVENTION

[0004] Reactive multilayer coatings are useful in a wide variety ofapplications requiring the generation of intense, controlled amounts ofheat in a planar region. Such structures conventionally comprise asuccession of substrate-supported coatings that, upon appropriateexcitation, undergo an exothermic chemical reaction that spreads acrossthe area covered by the layers generating precisely controlled amountsof heat. While we will describe these reactive coatings primarily assources of heat for welding, soldering or brazing, they can also be usedin other applications requiring controlled local generation of heat suchas propulsion and ignition.

[0005] In many methods of bonding or joining materials, a heat source isrequired. This heat source may either be external or internal to thestructure to be joined. When external, the heat may be generated from adevice such as a furnace. Processes incorporating such heat sourcesrequire the heating of the entire unit to be bonded, including the bulkmaterials and the bond material, to a temperature high enough to meltthe bond material. Such a method presents problems because the bulkmaterials to be joined are often delicate or sensitive to the hightemperatures required in the process. These high temperatures may damagethe materials to be bonded.

[0006] To alleviate the problems associated with external heat sources,internal heat sources are utilized. These heat sources often take theform of reactive powders or even electrical wires. When reactive powdersare used, a mixture of metals or compounds that will reactexothermically in a self-propagating reaction to form a final compoundor alloy is utilized. Such processes have existed since self-propagatingpowders were developed in the early 1960s, spawning what is known asSelf-Propagating, High-Temperature Synthesis (SHS). SHS reactions,however, often require substantial preheating to self-propagate, andcontrolling the rate and manner in which their energy is released isoften difficult. As a result, bonding may be inconsistent orinsufficient.

[0007] To combat the problems associated with reactive powder bonding,multilayer structures comprised of materials, which allow similarexothermic reactions, have been developed. Such structures allow formore controllable and consistent heat generating reactions. The basicdriving force behind such SHS reactions is a reduction in atomic bondenergy. When a structure having a series of layers of reactive material(known as a foil) is ignited, heat is produced as the distinct layersatomically combine. This heat ignites adjacent regions of the foil,thereby allowing the reaction to travel the entire length of thestructure, generating heat until all material is reacted. Even with suchadvances in bonding technology, however, there remain problems. Manymaterials, for example, posed major difficulties and previously couldnot be successfully bonded. Additionally, methods utilizing reactivefoils as heat sources often resulted in the foil debonding from thesubstrate upon reaction, thereby weakening the bond. Accordingly thereis a need for improved reactive multilayer foils.

SUMMARY OF THE INVENTION

[0008] Reactive foils and their uses are provided as localized heatsources useful, for example, in ignition, joining and propulsion. Animproved reactive foil is preferably a freestanding multilayered foilstructure made up of alternating layers selected from materials thatwill react with one another in an exothermic and self-propagatingreaction. Upon reacting, this foil supplies highly localized heat energythat may be applied, for example, to joining layers, or directly to bulkmaterials that are to be joined. This foil heat-source allows rapidbonding to occur at room temperature in virtually any environment (e.g.,air, vacuum, water, etc.). If a joining material is used, the foilreaction will supply enough heat to melt the joining materials, whichupon cooling will form a strong bond, joining two or more bulkmaterials. If no joining material is used, the foil reaction suppliesheat directly to at least two bulk materials, melting a portion of eachbulk, which upon cooling, form a strong bond. Additionally, the foil maybe designed with openings that allow extrusion of the joining (or bulk)material through the foil to enhance bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Many advantages, features, and applications of the invention willbe apparent from the following detailed description of preferredembodiments of the invention, which is provided in connection with theaccompanying drawings. In the drawings:

[0010]FIG. 1 illustrates an exemplary multilayer reactive foil duringreaction;

[0011]FIG. 2 shows the freestanding elements of an exemplary joiningapplication;

[0012]FIG. 3 illustrates initiation of a joining application;

[0013]FIG. 4 shows an exemplary perforated reactive foil;

[0014]FIG. 5 depicts the flow of joining material through holes in afoil;

[0015]FIG. 6 illustrates formation of a reactive foil by cold rolling;

[0016]FIG. 7 is a schematic cross section of a composite reactive foilcomposed of sets of microlaminate foils and nanolaminate foils;

[0017]FIG. 8 shows the use of reactive foil to join a semiconductor ormicroelectronic device to a substrate; and

[0018]FIGS. 9A and 9B illustrate a patterned reactive foil wherein someregions react to form conductive regions and others form non-conductiveregions.

[0019] It is to be understood that these drawings are for the purpose ofillustrating the concepts of the invention and are not to scale.

DETAILED DESCRIPTION

[0020] Preferred embodiments and applications of the invention will nowbe described. Other embodiments may be realized and compositional orstructural changes may be made without departing from the spirit orscope of the invention. Although the embodiments disclosed herein havebeen particularly described as joining or bonding bulk materialsutilizing a freestanding, self-propagating reactive foil structure, itshould be readily apparent that the invention may be embodied for otheruses or applications requiring an intense localized heat source.

[0021] In accordance with a preferred embodiment of the invention, amultilayer reactive structure (generically referred to herein as a“foil”) is provided as a local heat source in a variety of applicationssuch as a process for joining two or more (of the same or different)materials together. As illustrated in FIG. 1, reactive multilayer foil14 is made up of alternating layers 16 and 18 of materials A and B,respectively. These alternating layers 16 and 18 may be any materialsamenable to mixing of neighboring atoms (or having changes in chemicalbonding) in response to a stimulus, including silicides (e.g., Rh/Si,Ni/Si, and Zr/Si, etc.), aluminides (e.g., Ni/Al, Ti/Al, Monel/Al, andZr/Al, etc.), borides (e.g. Ti/B), carbides (e.g., Ti/C), thermitereacting compounds (e.g., Al/Fe₂O₃ or Al/Cu₂O), alloys, metallicglasses, and composites (e.g., metal ceramic).

[0022] The materials (A/B) used in fabrication of the reactive foil arepreferably chemically distinct. In a preferred embodiment, layers 16, 18alternate between a transition metal (e.g., Ti, Ni, etc.) and a lightelement (e.g., B, Al, etc.). Preferably, the pairs (A/B) of elements arechosen based on the way they react to form stable compounds with largenegative heats of formation and high adiabatic reaction temperatures, asdescribed in T. P. Weihs, “Self-Propagating Reactions in MultilayerMaterials,” Handbook of Thin Film Process Technology, 1997, which isincorporated herein by reference in its entirety. In a preferredembodiment, at least one of the layers of the reactive foil is (orcontains) Al.

[0023] When a multilayer foil 14 is exposed to a stimulus (e.g., spark,energy pulse, etc.), for example at one end, neighboring atoms frommaterials A and B mix, e.g. as shown in region 30. The change inchemical bonding caused by this mixing results in a reduction in atomicbond energy, thus generating heat in an exothermic chemical reaction.This change in chemical bonding occurs as layers with A-A bonds (i.e.,layer 16) and layers with B-B bonds (i.e., layer 18) are exchanged forA-B bonds, thereby reducing the chemical energy stored in each layer,and generating heat. As FIG. 1 further illustrates, this generated heatdiffuses through foil 14 (in a direction from reacted section 30 throughreaction zone 32 to unreacted section 34) and initiates additionalmixing of the unreacted layers. As a result, a self-sustaining/selfpropagating reaction (SHS reaction) is produced through foil 14. Withsufficiently large and rapid heat generation, the reaction propagatesacross the entire foil 14 at velocities typically greater than 1 m/s. Asthe reaction does not require additional atoms from the surroundingenvironment (as, for example, oxygen in the case of combustion), thereaction makes foil 14 a self-contained source of energy capable ofemitting bursts of heat and light rapidly, reaching temperatures above1400 K, and a local heating rate reaching 10⁹ K/s. This energy isparticularly useful in applications (e.g., joining, ignition, etc.)requiring production of heat rapidly and locally.

[0024] When a reaction propagates across a multilayer foil 14 asillustrated by FIG. 1, the maximum temperature of the reaction istypically located at the trailing edge of the reaction zone 32. This maybe considered the final temperature of reaction, which can be determinedby the heat of reaction (ΔH_(TX)), the heat lost to the environment orsurrounding material (ΔH_(env)), the average heat capacity of the sample(C_(p)), and the mass of the sample, M. Another factor in determiningthe final temperature is whether or not the reaction temperature exceedsthe melting point of the final product. If the melting point isexceeded, then some heat is absorbed in the state transformation fromsolid to liquid of the product. The final temperature of reaction may bedetermined using the following formulas (where T_(o) is the initialtemperature, ΔH_(m) is the enthalpy of melting, T_(m) is the meltingtemperature of the product, and there is no reaction with thesurrounding environment or material), depending upon whether finalproduct melting occurs:

T _(f) =T _(o)−(ΔH _(rx) +ΔH _(env))/(C _(p) M)

[0025] If no melting of final product occurs;

T _(f) =T _(m)

[0026] If there is a two-phase region of solid and liquid final product;and

T _(f) =T _(o)−(ΔH _(rx) +ΔH _(env) +ΔH _(m))/(C _(p) M)

[0027] If the final product completely melts.

[0028] Intricately related to the heat of the foil reaction is thevelocity of the propagation of the reaction along the length of foil 14.The speed at which the reaction can propagate depends on how rapidly theatoms diffuse normal to their layering (FIG. 1) and how rapidly heat isconducted along the length of foil 14. The propagation velocity is astrong function of the thicknesses of the individual layers in themultilayer foil. As the thickness of individual layers 16, 18 decreases,the diffusion distances are smaller and atoms can mix more rapidly. Heatis released at a higher rate, and therefore the reaction travels fasterthrough the foil structure.

[0029] In accordance with a preferred embodiment of the invention,reactive multilayer foils 14 may be fabricated by physical vapordeposition (PVD) methods. A magnetron sputtering technique, for example,may be used to deposit the materials A/B on a substrate (shown in FIG. 1in dashed outline form as layer 35) as alternating layers 16, 18.Substrate 35 may be rotated over two sputter guns in a manner well knownin the art to effectuate the layering of materials A/B into alternatinglayers 16, 18.

[0030] Substrate 35 is shown in dashed outline form to indicate that itis a removable layer that facilitates fabrication of the reactive foil14 as a freestanding foil. Substrate 35 may be any substrate (e.g., Si,glass, or other underlayer) having the characteristics of providingsufficient adhesion so as to keep the foil layers on the substrateduring deposition, but not too adhesive to prevent the foil from beingremoved from the substrate following deposition. The substrate caninclude a coating of release material or adhesion material to fine tuneits adhesion characteristics.

[0031] Advantageously an additional wetting layer (e.g., tin) may beused as an interface layer between the first layer of foil (16 or 18)and the substrate 35 to provide the necessary adhesive. When no wettinglayer is employed, selection of the appropriate material A/B as thefirst layer deposited on the substrate will ensure that the necessaryadhesive requirements are met. When a reactive foil using Al/Monel asmaterials A/B is to be fabricated, for example, without a wetting layer,the exemplary reactive foil would be deposited on a substrate such as Siwith the first layer being Al deposited on the substrate. Al ispreferably selected as the first layer in such case because Al willsufficiently adhere to Si during depositing, but will allow peeling offof the substrate after the foil is formed.

[0032] A fabricated foil 14 may have hundreds to thousands ofalternating layers 16 and 18 stacked on one another. Individual layers16 and 18 preferably have a thickness ranging from 1-1000 nm. In apreferred embodiment, the total thickness of foil 14 may range from 10μm to 1 cm.

[0033] Another method of fabricating is to deposit material in acodeposition geometry. Using this method, both material sources aredirected onto one substrate and the atomic fluxes from each materialsource are shuttered to deposit the alternate layers 16 and 18. Analternative method is to eliminate shuttering altogether and rotatesubstrates over two material sources that have physically distinctatomic fluxes. With this method, each pass over a source preferablygenerates an individual layer.

[0034] Preferably the degree of atomic intermixing of materials A/B thatmay occur during deposition should be minimized. This may beaccomplished by depositing the multilayers onto cooled substrates,particularly when multilayers 16 and 18 are sputter deposited. To theextent that some degree of intermixing is unavoidable, a relatively thin(as compared to the alternating unreacted layers) region of pre-reactedmaterial 20 will be formed. Such a pre-reacted region 20, nevertheless,is helpful in that it serves to prevent further and spontaneous reactionin foil 14.

[0035] In an alternative embodiment, a multilayer reactive foil may befabricated using mechanical techniques such as repeated rolling oflayered composites.

[0036] As illustrated in FIG. 1, the preferred reactive foil 14 is afreestanding multilayer reactive foil for particular use as aheat-generating source. Freestanding foils are easier to characterizethan thin films because they can be handled like “bulk” samples. Makingreactive foils 14 freestanding greatly expands their possible uses.Because such reactive foils are not necessarily associated with anyparticular application, they may be mass-produced for any purposerequiring a self-propagating localized heat source. Their production isnot limited or impeded by placing large or delicate items into a vacuumchamber to be coated by a reactive multilayer foil. Moreover,freestanding foils will allow heat sinking to the substrate to beminimized where unwanted.

[0037] Freestanding foils in accordance with preferred embodiments ofthe invention may be adapted for use in a variety of applications. Forexample, the freestanding foils may be used to couple bodies ofmaterials (referred to herein as “bulk materials”) together to form aunified product. Freestanding foils may find use in any number ofbonding, soldering, brazing, welding or other applications to join bulkmaterials. A typical joining application is represented in FIG. 3, inwhich two or more bulk materials 10 are to be joined together. The bulkmaterials 10 may be ceramics, metallic glasses, metals/alloys, polymers,composites, semiconductors, and other forms of material.

[0038] In the particular joining application illustrated in FIG. 3,joining material 12 is used to join bulk materials 10 together. Joiningmaterial 12 may be any layer (or composite layer) of material to bemelted to join bulk materials 10 together. Joining material 12 can be inthe form of freestanding sheets made up of metallic glasses,metals/alloys, functionally graded layers, Ni—B films, solder, brazes,self-propagating braze, combinations of such, or other like joiningmaterials.

[0039] In accordance with a preferred embodiment of the invention, areactive foil 14 is positioned between joining materials 12 to form astructure somewhat like a sandwich. The reactive foil “sandwich” thusformed is preferably positioned between bulk materials 10 at thelocation (e.g., end point, joint, intersection, etc.) at which the bulkmaterials 10 are to be joined together.

[0040] Alternatively, a reactive foil 14 is positioned between bulkmaterials 10 which have previously been coated with joining materials12.

[0041] As another alternative, a reactive foil 14 is positioned betweenjoining materials 12 to form a structure somewhat like a sandwich. Thereactive foil “sandwich” thus formed is preferably positioned betweenbulk materials 10 at the location (e.g., end point, joint, intersection,etc.) at which the bulk materials 10 are to be joined together. The bulkmaterials are first coated with joining materials.

[0042] The joining process involves the application of force (assymbolically represented by vice 11 in FIG. 3) to maintain the relativepositions of bulk materials 10, joining materials 12, and reactive foil14. Advantageously all components are freestanding elements pressedtogether. In an alternative embodiment, joining materials 12 are pressedas a composite with reactive foil 14.

[0043] Once the components of the joining process are positioned, astimulus (shown as lighted match 15) is applied, preferably, to one endof reactive foil 14 to initiate an SHS reaction. The intermixing ofatoms within reactive foil 14 produces rapid and intense heat sufficientto melt joining materials 12 along the entire length of reactive foil14. In this state, joining materials 12 are sufficient to join bulkmaterials 10 together. Shortly thereafter, the joined materials 10return to the temperature of the environment (e.g., room temperature)and can be removed from the applied force (graphically represented byvice 11).

[0044] A composite structure composed of joining materials 12 andreactive foil 14 can be formed through deposition (e.g., vapordepositing) of reactive foil 14 onto one layer of joining material 12.Another layer of joining material is then combined with reactive foil 14through vapor deposition or an application of force (e.g., coldrolling).

[0045] Advantageously a wetting/adhesion layer may be added tofacilitate surface wetting for the reactive foil 14, bulk materials 10,or both. The wetting/adhesion layer allows uniform spreading of joiningmaterial to ensure consistent joining of bulk materials. Thewetting/adhesion layer may be a thin layer of joining material (e.g.,braze), Ti, Sn, metallic glass, etc. Commercial alloys such as Ag—Sn,Ag—Cu—Ti, Cu—Ti, Au—Sn, and Ni—B may also be used.

[0046] Preferred embodiments of the invention are useable asfreestanding reactive foils 14 with increased total thickness. The totalthickness of such a reactive foil depends upon the thickness and numberof the elemental layers (e.g., 16 and 18) utilized to form the foils.Foils that are less than 10 μm are very hard to handle as they tend tocurl up on themselves. Foils on the order of 100 μm are stiff, and thus,easily handled. Thicker foils also minimize the risk of aself-propagating reaction being quenched in the foils. In joiningapplications using reactive foils, there is a critical balance betweenthe rate at which the foil generates heat and the rate at which thatheat is conducted into the surrounding braze layers and the joint to beformed. If heat is conducted away faster than it is generated, thereaction will be quenched and the joint cannot be formed. The thickerfoils make it harder to quench the reaction because there is a largervolume generating heat and the same surface area through which heat islost.

[0047] Thicker foils can be utilized with reaction temperatures that arelower, generally leading to more stable foils. Foils with high formationreaction temperatures are generally unstable and brittle and thereforeare dangerous and difficult to use. Brittle foils, for example, willcrack easily, leading to local hot spots (through the release of elasticstrain energy and friction) that ignite the foil. Cutting such brittlefoils (e.g., for specific joint sizes) is very difficult to do as theyare more likely to crack into unusable pieces or igniting during thecutting process. Freestanding thick foils offer the advantage ofovercoming problems associated with thermal shock and densificationproblems that have presented obstacles in known processes. Bothphenomena relate to rapid changes in the dimensions of the foils. Onreacting, the foils will heat rapidly and will try to expand beyond thesubstrate that constrains them. This leads to a thermal shock and foilsthat are deposited on substrates can debond, thereby causinginconsistent and less effective bonding. As the reaction proceeds, thefoils will also density, due to the change in chemical bond. Thisdensification, can also cause debonding from a substrate andinconsistent and ineffective bonding. By making the foil freestanding inaccordance with a preferred embodiment of the invention, no debondingoccurs, the foil is easily manipulated and handled, and thus thereactive foil is made available to a greater variety of applications.

[0048] In accordance with a preferred embodiment, the thicker reactivefoils are on the order of 50 μm to 1 cm thick. Although a number ofdifferent systems may be employed to create the thick freestandingreactive foils, a unique process in selecting the fabrication conditionsfor the employed system should be carefully selected. For example,deposition conditions such as sputter gas and substrate temperature areadvantageously chosen so that stresses remain sufficiently low in thefilms of the foil as they are grown in the system. Since the stress inthe film times its thickness scales with the driving force fordelamination, the product of stress and thickness should be kept below1000 N/m. Stresses often arise in the films during the fabricationprocess. As the films grow thicker, they are more likely to peel offtheir substrates or crack their substrates than thinner films, therebyruining the final foil production. By characterizing the stresses in thefilms and selecting conditions to minimize the stresses, the fabricationprocess can be completed without the premature peeling off of the foilor the cracking of the substrate.

[0049] In an alternative embodiment, openings in a reactive foil areintentionally designed in the foil structure. These openings are ofparticular use in facilitating and enhancing joining applications.

[0050] The foil may be initially fabricated, for example, with one ormore openings or perforations 22 through the foil structure, as shown inFIG. 4. Preferably the openings are formed in a periodic pattern, suchas a rectangular array, across the foil area. Any known method may beemployed to create openings. For example, sputter depositing of the foil14 on a removable substrate with patterned holes may be used. Theopenings may also be formed by depositing the foil 14 onto a substrate,depositing photoresist on the foil, patterning the photoresist withphotolithography, and then etching the underlying foil through thepatterned holes. A further exemplary technique involves physicallypunching holes in foil 14. Preferably the openings have effectivediameters in the range of 10-10,000 micrometers. (The effective diameterof a non-circular opening is the diameter of a circular opening of equalarea.)

[0051] As shown in FIG. 4, the openings in foil 14 allow joiningmaterial 12, or bulk material 10 in some circumstances, to extrude (asshown by arrows 26) through these perforations 22 upon being heated andmelted by the exothermic reaction of foil 14. Upon this extrusion, onelayer of joining material 12, or bulk material 10, may contact andcouple with another layer 12, or bulk material 10, on the opposite sideof the freestanding foil 14. The patterned perforations 22 permitenhanced bonding of bulk materials 10 to reactive foil 14 and each othermaking stronger and more consistent bonds.

[0052]FIG. 5 is a microphotograph showing two copper bodies 53, 54bonded by silver solder 50 that has extruded through openings, e.g. 51in a reacted foil 52.

[0053] Utilizing one or more embodiments of the invention, a number ofdifferent applications can now be performed more effectively andefficiently. For example, metallic glass bulk materials can now bejoined, where the end product is a single structure made up solely ofmetallic glass, including the bond and reacted foil layer. It is alsonow possible to join bulk materials with very different chemicalcompositions, thermal properties, and other physical properties, thathistorically presented many difficulties in bonding. Semiconductor ormicroelectronic devices may be bonded to circuit boards or otherstructures, and at the same time, multiple leads may be created that areintricately associated with the devices. Semiconductor andmicroelectronic devices may also be sealed hermetically.

[0054] These joining applications are enhanced by the invention in thatpotential for heat damage, normally associated with such applications assoldering, brazing, and welding, is avoided or at least minimized.

[0055] Moreover, utilizing embodiments of the invention, the bulkmaterials being joined may be freestanding. This means that prior to theactual joining of the bulk materials, the individual bulk substrates donot need any braze layer deposited directly upon them. Additionally, thebulk substrates do not necessarily require any pre-bonding of thereactive foil or other pre-treatment. The bulk materials involved maysimply be held securely to either a freestanding braze layer or thefreestanding reactive foil at the time of bonding for a strong andpermanent joint to be created.

[0056] Embodiments of the invention allow bonding at least one bulk thatis a metallic glass. No braze need be associated with that bulk in thejoining process. This is because the reactive foil may be designed tobond directly with a metallic glass upon reaction. To accomplish thisjoining process, the reactive foil can itself react to form a metallicglass.

[0057] Embodiments of the invention further allow for superior bondingwhen the bulk materials include microchips or semiconductor devices. Inthe bonding of semiconductor devices to a substrate such as a circuitboard, potential for damage to the device is a factor that must be takeninto consideration. By using a freestanding reactive foil to join such asemiconductor device to a substrate, little heat is generated that canbe damaging to the device or to adjacent components. The semiconductordevices may be situated on the substrate with greater freedom and ease.As described below, specific foil compositions, such as Ni/Al orMonel/Al, may be utilized. Foils of such composition are not only fareasier to handle than those of the past, but the combination of Ni, Cuand Al enables freestanding foils to have a high thermal and electricalconductivity.

[0058] When bonding is directed to bulk materials such as semiconductordevices, the reactive foil may have composition patterning properties.The embodiments allow the fabrication of alternating adjacentelectrically insulating and conducting regions in the final reactedfoil, thereby allowing a multitude of leads to be bonded andelectrically isolated with a single reaction.

[0059] In a preferred embodiment of the invention, less energy isrequired to perform a joining application utilizing reactive foils.Functionally graded layers as joining material allow for control overmelting temperatures through selection of their composite materials.Functionally graded layers may be utilized, for example, because theirmelting temperature may be controlled. Ni—B films used as joiningmaterial allow for low temperature melting where the melting temperaturebegins at a relatively low temperature and elevates as B diffuses out ofNi, resulting in a final material with a relatively high melting point.By requiring less energy from the foil reaction, the overall heatapplied to the total structure to be bonded can be reduced, therebyminimizing damage to the materials to be bonded due to the foilreaction.

[0060] In another embodiment, one may include layers of reactivemultilayer braze within the reactive multilayer foil. For example, in afoil comprising reactive layers of Al and reactive layers of Ti, Zr orHf alloys, one may include one or more reactive braze layers comprisinga Cu or a Ni alloy. The reactive multilayer braze would provide anenergy source as the layers mix and form the joining material, inaddition to the energy provided by the reactive foil. The combination ofreactive multilayer foil and reactive multilayer brazes permits the useof reactive brazes that may not self-propagate without the foil.

EXAMPLES

[0061] The invention may now be more clearly understood by considerationof the following specific examples:

Example 1

[0062] Reactive foils of Al and Ni are formed by cold rolling many 5 μmsheets of Ni and Al that are stacked together. FIG. 6 schematicallyillustrates fabrication of the foil 60 by passage of the stack 61between rollers 62A and 62B. The sheets can be cold-rolled several timesuntil the layers are reduced to the desired thickness.

Example 2

[0063] Instead of utilizing foils comprised of multilayers of uniformthickness, a composite foil is used, in which nanolaminate reactivemultilayers are deposited onto reactive microlaminate foils. Asillustrated in FIG. 7 certain sections of layers 70 within the reactivefoil 71 will be of a nanoscale (nanolaminate), while other sections 72will be of micron-thick layers (microlaminate). The nanolaminate, asdescribed above, will react easily and the reaction, once started, willself-propagate along the length of the foil without being quenched bythe melting of the surrounding braze layers or bulk components. Thus,the nanolaminate can be viewed as an igniter for the microlaminate. Thesection 72 with microscale layers may not be able to sustain aself-propagating reaction at room temperature, but when heated byadjacent nanolaminate sections 70, it will sustain such a reaction. Thefoil can comprise alternate layers of Al and Ni.

Example 3

[0064] In fabricating these composite foils, sheets of Al and Ni arerolled to form the microlaminate section and then a nanolaminate foil isvapor deposited onto either side of this microlaminate structure.Fabrication may also be performed through vapor deposition of the fullcomposite with the microlaminate layers deposited at much higher rateswithout igniting the foil or causing unacceptable intermixing betweenthe alternating layers during deposition.

Example 4

[0065] A reactive multilayer braze is formed that is similar to thereactive foils described above, which reacts to form a metallic glass.This multilayer braze gives off heat upon a reaction of its alternatinglayers. Through a careful selection of reactants that are know to begood glass formers, the braze will form an amorphous final product uponreaction, similar to those in commercial use and to the foils describedabove. The heat generated by the reacting braze layers reduces theamount of reactive foil required for joining.

Example 5

[0066] Certain compositions of foil 14 may react to form amorphousalloys (metallic glass). Those foils may be combinations of layers ofalloys that comprise Ni or Cu, alloys that comprise Ti, Zr, or Hf, andalloys that comprise Al as such will react to form metallic glass. Whenusing such foils, certain properties may be attained. Metallic glasseshave very good wetting capabilities. The braze layer may be excludedwhen using such a foil to join bulk metallic glass and in such acircumstance, once the foil reacts and is joined with the metallicglass, a single bulk metallic glass may be produced.

Example 6

[0067] A semiconductor or microelectronic device is joined to asubstrate such as a printed circuit board using a reactive multi-layerfoil. FIG. 8 schematically illustrates the joining arrangement whereinthe reactive foil 80 is sandwiched between solder layers 81A and 81B,and the sandwich is disposed between the contact lead 82 for the device83 and the contact surface 84 of the board.

Example 7

[0068] A patterned reactive foil is designed so that some sections reactto form electrically conductive regions and other sections formnon-conductive regions.

[0069]FIGS. 9A and 9B schematically illustrate the concept. FIG. 9Ashows the foil 90 before reaction. FIG. 9B is after the reaction.Regions 91, comprising alternate layers of Al and Ni, react to formconductive regions 95. Regions 92, comprise insulators, such as SiO₂ orsilicon nitride, or alternate layers that react to form non-conductiveregions.

[0070] It is contemplated that in use, regions 91 would be registeredbetween contacts above and below the foil 90 to be electricallyconnected through the regions 95 after the reaction.

[0071] It can now be seen that one aspect of the invention is a methodof making a reactive multilayer foil composed of a plurality ofalternating layers that can react exothermically. The method comprisesthe steps of providing a substrate, vapor depositing the alternatinglayers on the substrate to form the multilayer foil, and separating themultilayer foil from the substrate. Advantageously the substrate hassufficient adherence to the deposited layers to retain the layers duringdeposition but insufficient adherence to prevent removal of themultilayer foil after deposition. As an example, the layers can compriseone or more layers of aluminum deposited in contact with a siliconsubstrate. Alternatively, the substrate can include a coating of releasematerial or an adhesion material to achieve the proper level ofadherence.

[0072] One approach for separating the multilayer foil from thesubstrate is to provide a substrate with a sacrificial layer (or makethe entire substrate a sacrificial layer) that can be etched or peeledaway from the foil after deposition. Exemplary materials for asacrificial layer include copper, brass and photoresist.

[0073] The vapor depositing of the layers is preferably by physicalvapor deposition such as by magnetron sputtering or electron beamevaporation. Advantageously the substrate is cooled during the vapordepositing to reduce intermixing of the alternating layers, to reduceenergy losses and to reduce stresses in the deposited layers.Advantageously the layers are deposited to form a multilayer foil havinga thickness in the range 50 μm -1 cm. Foils thus made with a thicknessin the range 10 μm to 1 cm can be used as freestanding foils.

[0074] Another aspect of the invention is a method of bonding a firstbody to a second body comprising the steps of providing a freestandingreactive multilayer foil, pressing the bodies together against the foiland igniting the reactive foil. The ignited foil can melt material ofthe bodies or melt an associated meltable (joining material) layer tojoin the bodies together. Alternatively, the reaction product of thelayers can itself be the joining material. One or both of the bodies canbe semiconductor or microelectronic devices. The method is particularlyadvantageous for joining bodies having coefficients of thermalexpansion, which differ by 1 μm/m/° C. or more.

[0075] In an alternative embodiment, a reactive multilayer foil includesa plurality of openings through the thickness of the foil. The openingsare preferably periodic over the foil area. These openings can be leftunfilled or they can be filled with meltable materials, propellants, orother materials that will change or react on heating when the reactivefoil is ignited.

[0076] Such apertured foils can be made by providing a substrate havinga surface including a plurality of preformed openings, bumps orparticles of thickness (or depth) comparable to or larger than thethickness of the multilayer foil to be deposited, depositing thereactive multilayer foil, and separating the resulting aperturedmultilayer foil from the substrate.

[0077] Alternatively, a reactive multilayer foil can be deposited on asubstrate, patterned by a removable masking layer, and etched to form aplurality of holes. The apertured foil can then be removed from thesubstrate. Yet further in the alternative, a continuous foil can beformed and holes can be formed in the continuous foil by mechanicalpressing.

[0078] The apertured foils have an important application in bonding. Areactive foil perforated by a plurality of openings is disposed betweena first and a second body. If the body material is not meltable by thefoil, a separate meltable layer or coating of meltable joining materialis also disposed between the bodies. The bodies are pressed togetheragainst the foil (and joining material) and the foil is ignited to meltjoining material. The melted material flows within and through theopenings in the foil to join the bodies. This approach produces acharacteristic joint with ductility enhancing bridges through theopenings. It is especially advantageous where one or both bodies aresemiconductor or microelectronic devices or where the devices have CTEsthat differ by more than 1 μm/m/° C.

[0079] A third type of novel reactive foil is a composite reactivemultilayer foil in which the individual layers in the foil differ inthickness or in composition, on moving across the total thickness of thefoil, to achieve advantageous results. One exemplary composite reactivemultilayer foil comprises a first section with a plurality of relativelythick reactive layers, e.g. 1 μm to 10 μm, stacked on a second sectionwith a plurality of thinner reactive layers(e.g. 1-1000 nm). The sectionwith the thinner reactive layers ignites more rapidly than would thesection with the thicker reactive layers. But as ignition spreads acrossthe thinner section, it ignites the thicker section to produce a moreuniform ignition and higher heat. Similar results can be achieved byvariation of the foil composition in the thickness direction.Compositional variations can provide one set of layers whose reactionproduct provides joining material and another set of more reactivelayers for igniting the first set. Compositional variations can beachieved, for example, by varying the process parameters in vapordepositing in accordance with techniques well known in the art.

[0080] A fourth type of novel reactive foil has a major surface areacomposed of at least two different regions: one or more first regionswhich will react to form electrically conductive material and one ormore second regions which are non-conductive. Such foils areparticularly useful in connecting semiconductor device electricalcontacts to a substrate having receiving contacts. A foil can bedisposed between the device and the substrate with the device contacts,the contacts of the substrate and the first regions of the foil all inregistration. The device and substrate are then pressed against thefoil, and the foil is ignited to bond the device to the substrate withthe respective contacts conductively connected and the other regionsnon-conductively bonded.

[0081] While preferred embodiments of the invention have been describedand illustrated, it should be apparent that many modifications to theembodiments and implementations of the invention can be made withoutdeparting from the spirit or scope of the invention. While theillustrated embodiments have been described generically referring to thejoining of bulk materials, it should be readily apparent that anymaterials that are to be coupled (permanently or temporarily) togetherthrough soldering, brazing, welding or other known technique can becoupled together utilizing the invention. Materials such as metallicglasses (e.g., amorphous glass), metals (e.g., Cu) and alloys (e.g.,stainless steel), polymers, ceramics (e.g. SiC), composites,semiconductors, and numerous others in various combinations. The scopemade available as a direct result of the advantages derived by joiningmaterials utilizing the invention range from large scale bonding of SiCarmor to Ti—Al—V tank bodies, or the bonding of carbide coatings to toolbits, to microscopic bonding of microchips to circuit boards on a nanoor microscale.

[0082] The stimulus used to initiate the self-sustaining reaction in thereactive foils of the preferred embodiments may be any form of energysuch as the impact from a sharp stylus, spark from an electrical source,heat from a filament, radiation from a laser, etc. Although theillustrated embodiments have been described as applied in an environmentof air at room temperature, it should be readily apparent that theinvention may be practiced in other environments including vacuum,argon, water, etc.

[0083] It should be readily apparent that the quantitative data (e.g.,reaction velocity, peak temperature, heating rate, etc.) of particularembodiments may easily be modified by varying elements of the reaction(e.g., varying composition of materials A or B, thickness of layers,total thickness of foil, or braze layer composition/thickness).

[0084] Although the embodiments specifically illustrated herein depictjoining materials in the form of two sheets forming a sandwich around areactive foil (as shown, for example, by sheets 12 and foil 14 in FIGS.2 and 3), it should be apparent that any number of sheets (or otherstructures) of joining materials may be used, including a single layerwrapped around reactive foil 14 or joining materials attached to thebulk components. In accordance with a preferred embodiment, no layer ofjoining material at all may also be used. For example, metallic glassbulk material, metallic glass reactive foils, or both may be used injoining applications without the use of joining material (e.g., braze).

[0085] Moreover, although the illustrated embodiments have only utilizedtwo different materials A/B as alternating layers in a reactive foil, itshould be apparent that any number of material layers can be utilized toform a reactive foil in accordance with the invention.

[0086] In addition, many of the preferred embodiments disclosed herein(e.g., patterning foils, perforations in the foil, etc.) make particularuse of freestanding foils, it should be readily apparent, however, thatthese embodiments and other aspects of the invention may be implementedwithout use of freestanding foils. Furthermore, it should be readilyapparent that the intentionally designed openings in the reactive foilsurface may penetrate through any number of layers in the foil, althoughit is preferred that the entire foil structure be penetrated as shown,for example, in FIG. 5. The openings, while depicted in FIG. 4 ascircle-shaped holes 22, may be any single (or combination) of shapesforming one or more patterned structures on the reactive foil. Theopenings may be formed vertically in the direction normal to the layersof the reactive foil, or be formed at one or more angles through thelayers of the foil.

[0087] Thus numerous and varied other arrangements can be made by thoseskilled in the art without departing from the spirit and scope of theinvention.

What is claimed:
 1. A method of making a freestanding reactive multilayer foil composed of a plurality of alternating layers that can react exothermically, comprising the steps of: providing a substrate; vapor depositing the alternating layers on the substrate to form the reactive multilayer foil; and separating the multilayer foil from the substrate.
 2. The method of claim 1 wherein the substrate has sufficient adherence to the deposited layers to retain the layers during deposition but insufficient adherence to prevent removal of the multilayer foil after deposition.
 3. The method of claim 1 wherein the layers comprise one or more layers of aluminum, and at least one of the layers of aluminum is deposited in contact with the substrate.
 4. The method of claim 3 wherein the substrate comprises silicon.
 5. The method of claim 1 wherein the substrate comprises a coating of a release material or a coating of an adhesion material.
 6. The method of claim 1 wherein the substrate comprises a removable sacrificial layer.
 7. The method of claim 1 wherein the substrate comprises a removable sacrificial layer of copper, brass or photoresist.
 8. The method of claim 1 wherein the vapor depositing comprises physical vapor deposition.
 9. The method of claim 8 wherein the vapor depositing comprises magnetron sputtering or electron beam evaporation.
 10. The method of claim 1 wherein the substrate is cooled during the vapor depositing.
 11. The method of claim 1 wherein the layers are deposited to form a multilayer foil having a thickness in the range 50 μm-1 cm.
 12. The method of claim 1 wherein the vapor depositing is under conditions chosen to minimize stress in the deposited layers.
 13. A method of bonding a first body to a second body comprising the steps of: disposing between the first body and the second body, a freestanding reactive multilayer foil; pressing the bodies together against the foil; and igniting the reactive foil.
 14. The method of claim 13 wherein at least one of the bodies is a semiconductor or microelectronic device.
 15. The method of claim 13 wherein the reactive multilayer foil has a thickness in excess of 10 μm.
 16. The method of claim 13 wherein the bodies have coefficients of thermal expansion differing by at least 1 μm/m/° C.
 17. The method of claim 13 wherein the first body comprises metal and the second body comprises ceramic material.
 18. The product made by the method of claim
 13. 19. A reactive multilayer foil comprising: a foil composed of alternating layers that react exothermically, wherein the foil includes a plurality of openings through the foil.
 20. A reactive multilayer foil according to claim 19 wherein the openings are filled with meltable material, propellant, or material that changes or reacts on heating.
 21. A reactive multilayer foil according to claim 19 wherein the openings are periodically arranged across the area of the foil.
 22. A method of making a reactive multilayer foil comprising the steps of: providing a substrate having a surface including a plurality of preformed openings, bumps, or particles of thickness or depth similar to or greater than the multilayer foil to be deposited; depositing on the surface a plurality of layers to form the reactive multilayer foil; and separating the multilayer foil from the substrate.
 23. A method of making a reactive multilayer foil comprising the steps of: providing a flat substrate; depositing on the substrate a plurality of layers to form a reactive multilayer foil; depositing a masking layer on top of the reactive foil; patterning the masking layer with a plurality of holes; etching the reactive foil through the holes; and separating the multilayer foil from the substrate.
 24. A method of making a reactive multilayer foil comprising the steps of: providing a flat substrate; depositing on the substrate a plurality of layers to form a reactive multilayer foil; and mechanically pressing a plurality of holes into the reactive foil.
 25. A method of making a reactive multilayer foil comprising the steps of: making a reactive multilayer foil having a plurality of openings through the foil, and filling the openings in the multilayer foil with meltable material, propellant, or material that will change or react on heating when the reactive foil is ignited.
 26. A method of bonding a first body to a second body comprising the steps of: disposing between the first body and the second body, a reactive multilayer foil and at least one meltable joining material, the reactive multilayer foil having a plurality of openings through the thickness of the foil; pressing the bodies together against the foil and the joining material; and igniting the reactive foil to melt the joining material and permit the melted material to flow through the openings to join the first and second bodies.
 27. The method of claim 26 wherein at least one of first body or the second body comprise a semiconductor or a microelectronic device.
 28. The method of claim 26 wherein the first body and the second body have CTEs that differ by more than about 1 μm/m/° C.
 29. The product made by the method of claim
 26. 30. The product made by the method of claim
 27. 31. The product made by the method of claim
 28. 32. A composite reactive multilayer foil comprising: at least one first set of reactive layers; and at least one second set of reactive layers in thermal contact with the first set, the layers of the first set having thicknesses which are relatively larger than those of the second set, whereby the layers of the second set, upon ignition, ignite the thicker layers of the first set.
 33. A composite reactive multilayer foil comprising: a first set of reactive layers; and a second set of reactive layers in thermal contact with the first set, the layers of the first set having compositions which are relatively more reactive than the second set, whereby the layers of the first set, upon ignition, ignite the less reactive layers of the second set.
 34. A reactive multilayer foil comprising: a multilayer foil having an area composed of at least two different regions, one or more first regions composed of layers that can react exothermically to form electrically conductive material and one or more second regions which are non-conductive or react to form non-conductive material.
 35. A method of connecting a semiconductor or microelectronic device having one or more electrical contacts to a substrate having one or more receiving contacts, comprising the steps of: disposing between the device and the substrate a reactive multilayer foil composed of one or more first regions that can react exothermically to form electrically conductive regions and one or more second regions which are non-conductive or react to form non-conductive material; registering the contacts of the device, the contacts of the substrate and the first regions of the foil, pressing the device and the substrate together against the foil; and igniting the foil.
 36. A method for bonding a first body to a second body comprising the steps of: disposing between the first body and the second body, a reactive multilayer foil comprising a plurality of successive exothermic reactive layers that react to form a joining material; pressing the bodies together against the foil; and igniting the foil.
 37. The method of claim 36 wherein at least one of the first and second bodies comprise metallic glass.
 38. The method of claim 37 wherein the reactive multilayer foil comprises alternate layers of alloys that, after reaction and cooling, are amorphous.
 39. The method of claim 37 wherein the reactive multilayer foil comprises alternate layers of an alloy comprising Ni or Cu, an alloy comprising Ti, Zr, or Hf, and an alloy containing primarily Al.
 40. A method of bonding a first body to a second comprising the steps of: disposing between the first body and the second body, a freestanding reactive multilayer foil and at least one layer of meltable joining material; pressing the bodies together against the foil and joining material; and igniting the reactive foil to melt the joining material.
 41. The method of claim 40 wherein the joining material is coated on the foil.
 42. The method of claim 40 wherein the joining material is freestanding.
 43. A bonded structure comprising: a first body; a second body bonded to the first body by a joining region, the joining region comprising a reacted multilayer structure including a periodic array of openings therethrough, the structure embedded in a matrix of meltable joining material extending through the openings to join the first body and the second body. 